1. Field of the Invention
The present invention is directed to a process for preparing ionomers. More specifically, the present invention is directed to a process for preparing ionomers by contacting the polymer to be ionomerized with the reaction product of a vinyl acetate polymer and a hydroxide compound.
2. Background of the Prior Art
Ionomers are attractive polymers in that they provide the advantages of thermoplastic and thermosetting polymers. These polymers are formed by ionically cross-linking a polymer possessing carboxylic acid or diacid anhydride functionality with metal ions to produce an ionic polymeric salt.
These polymers are usually formed by reacting a polymer having acid functionality with a base, usually a metal hydroxide, acetate or the like. Unfortunately, inorganic bases, such as alkali metal hydroxides, are insoluble in polymers having carboxylic acid functionality. Thus, the degree of reaction between the base and the polymer is relatively low.
One means of overcoming this problem involves the use of a solvent for the polymer and the base. This expedient improves the degree of reaction between the polymer and the base. However, the recovery of the ionomer product from the solution adds new complexities to the process. Those skilled in the art are aware of the difficulties attendant the recovery of an ionic polymer from a solution. These include the energy requirement associated with the removal of the solvent as well as the safety and health problems resulting from handling the often toxic and inflammable solvents utilized in this process. In addition, environmental problems associated with the disposal of the spent solvent adds further difficulty to the challenges associated with this method.
In view of the problems associated with the use of a solvent system for improving contact between the polymer to be ionomerized and the base, an alternate process is oftentimes employed. In this process the polymer is melted and the base is dispersed therein. Obviously, this method is limited in its effectiveness due to the limited improvement in contact between the polymer and the base. As such, large expenditures of energy, spent in melting the polymer and mixing the dispersion, is required to completely ionomerize the polymer. Even so, only approximately 30% to 40% by weight of the base, necessarily provided in stoichiometric excess, is usually reacted. The unfortunate result of this poor conversion is formation of a dispersion of the base in the ionomer. As a result, the ionomer is hazy, containing visible particles of the base compound.
Thus, the melt dispersion method suffers from an important deficiency of the solution method, high energy requirements. In addition, the resultant ionomer product is usually characterized by the presence of unreacted hydroxide particles which impart a hazy appearance to the resultant ionomer.
An attempt to overcome the problems associated with the solution and melt dispersion methods is addressed in U.S. Pat. No. 4,666,988. This patent attempts to overcome problems associated with ionomerization by providing a uniform dispersion of a metal oxide in a carboxyl-containing ethylene copolymer. This is accomplished by making a masterbatch blend of the metal oxide in a polyethylene resin. This process, directed primarily to employing magnesium oxide as the metal oxide, preferably utilizes a linear low density polyethylene as the ethylene polymer dispersing agent. However, the process of the '988 patent does not involve the generation of an effective, in situ formed, base uniformly dispersed in a thermoplastic resin.
The above remarks establish the need in the art for a more effective process of ionomerizing polymers having acid functionality which process is more effective than the solvent and melt dispersion methods used in the ionomerization methods of the prior art.